A good minecraft mod?

Could you be more specific on what you are looking for?

There are hundreds of mods in dozens of different categories. Some are just UI changes, some add one or two humorous items, some make it a completely new game.

S.E.US mod makes minecraft realistic looking with rain puddles, reflections, realistic sun, etc. But you have to have a good graphics card in order to run it.

Shadow World

Pixlemon